A automatic power control loop comprises a photo diode used for sensing a light intensity of a laser diode to generate a feedback current, a switch selector used for selecting one among a plurality of predetermined currents according to a control signal generated by a controller, a transducer used for transferring a current different between the feedback current and the selected predetermined current into a load voltage, a comparator used for comparing the load voltage with a reference voltage to generate a comparison signal, a counter used for counting a count value according to the comparison signal and the control signal, and a laser diode driver used for generating a corresponding bias current in response to the count value to drive the laser diode. Thereby, the bias current will be adjusted within an allowable range, so that the light source of the laser diode can maintain a constant light intensity.